The present invention relates to a process for producing preoxidized fiber (flameproof fiber) spun yarns from acrylonitrile fiber tows.
It has been known that preoxidized fibers can be obtained by an oxidation treatment (flameproofing treatment) of acrylonitrile fiber bundles at from about 200.degree. to 300.degree. C. in an oxidizing atmosphere. They can be used as a precursor for carbon fibers (i.e., nonactivated carbon fibers having generally high mechanical strength, which are used as reinforcing materials) or activated carbon fibers or they can be used directly for uses utilizing heat resistance and flame resistance thereof.
In the case of producing spun yarns of carbon fibers or activated carbon fibers, a process comprising spinning of preoxidized fibers, and then processing in a carbonization step or an activation step has been adopted, because spinning becomes more difficult to carry out after carbonization or activation because of a lowering of the mechanical strength. Further, in the case of directly utilizing the preoxidized fibers, they are often used as spun yarns.
However, these preoxidized fibers are not crimped produced by conventional processes, and they have low elongation or low knot strength. Accordingly, they are very difficult to be processed in the spinning step after being processed in a crimping step. Particularly, in the case of using acrylonitrile fibers subjected to a preoxidation for uses other than carbon fibers, it is necessary to carry out a more violent (i.e., under a higher temperature and/or for a longer period of time) preoxidation than in the case of using for carbon fibers. That is, in the case wherein the preoxidized fibers as the precursor of activated carbon fibers are not sufficiently oxidized, the yield of the activated carbon fibers is low, and fibers having a large surface area are difficult to obtain. Further, in order to provide properties as heat resisting noncombustible materials, it is necessary to carry out a more violent preoxidation than is the case when using for carbon fibers. In this way, in the case of using as a precursor for preparing carbon fibers, the fibers can be subjected to the carbonization step even though they are in a combustible state.
In order to produce preoxidized fibers for activated carbon fibers or a provide heat resistance or flame resistance to such fibers, it is necessary that the oxidation treatment be carried out until the specific gravity of the fibers is from about 1.35 to 1.45 or so, under which conditions further spinning is difficult to carry out. The fibers subjected to the oxidation treatment as described above can be of course processed in the carbonization step.
Hitherto, a process for producing spun yarns of preoxidized fibers has been proposed in Japanese Patent Application (OPI)-3112/77. (The term "OPT" as used herein refers to a "published unexamined Japanese patent application".) According to this process, the spun yarns are obtained from acrylonitrile filament bundles (composed of 1000-16000 filaments) by a process comprising the steps of preoxidation, steam thermal crimping, stretch cutting, combing, gilling, and roving, or steps of comprising preoxidation, steam thermal crimping, and production of slivers by stretch cutting.
According to such a process, however, since the steam thermal crimping is carried out before stretch cutting, the fibers are damaged in the steam thermal crimping step, and short fibers or powders of fibers are formed during the stretch cutting step, to cause neps or slubs when produced spun yarns. Consequently, combing is necessary to carry out to remove neps and slubs. Further, a number of flies is caused (i.e., the fiber becomes to powder and scatters) during the steps, to lower the yield of yarn. Moreover, spun yarns having high quality are difficult to obtain by such a process.